Luminescent materials are among the classic ingredients of security inks or coatings. They convert energy of an exciting radiation of a given wavelength into emitted light of another wavelength. The exploited luminescent emission can lie in the UV range (below 400 nm), in the visible range (400–700 nm) or in the near to mid infrared range (700–2500 nm) of the electromagnetic spectrum. Certain luminescent materials can emit simultaneously at more than one wavelength. Most luminescent materials can be excited at more than one wavelength.
If the emitted radiation has a longer wavelength than the exciting radiation, one speaks of “Stokes” or “down-converting” luminescence. If the emitted radiation has a shorter wavelength than the exciting radiation, one speaks of “anti-Stokes” or “up-converting” luminescence.
Luminescence can be of two different types: fluorescence or phosphorescence. Fluorescence is the prompt emission of radiation upon excitation, whereas phosphorescence is the time-delayed emission of radiation, observable after the excitation has been stopped. Phosphorescence, also called afterglow, is characterized by a specific decay of the luminescence intensity in function of time; the corresponding lifetimes, which are material-specific, can range from the nanosecond to the multi-hour time scale.
Luminescent materials can be of organic or of inorganic nature. Examples of the former are the cyanine type molecules, as well as the coumarines, rhodamines etc. Examples of the latter are the copper or silver doped zinc sulfides, the rare-earth doped yttrium aluminum garnets or yttrium vanadates, etc. Another class of luminescents can be found among the metallo-organic compounds, e.g. the silicon phthalocyanines, the rare-earth beta-diketonates etc.
Luminescent materials can be employed in inks or coatings either as pigments or as soluble materials. Newer developments have also made available luminescent pigments in colloidal form. Particular applications rely as well on luminescent polymers, obtained by polymerizing, copolymerising or grafting luminescent molecules into or onto a polymer chain.
All of these compound classes and application forms have been used in security compositions and for security purposes. Corresponding detecting equipment can be made to discriminate between prompt luminescence (fluorescence) or delayed luminescence (phosphorescence).
U.S. Pat. No. 3,473,027 deals with the general use of organic and inorganic rare-earth compounds as visible and IR luminescent markers for applications such as goods identification and labels, personal identification, identification and registration of passing vehicles, machine reading of information, ZIP codes, invoices, tags etc. and high capacity storage devices. The patent further describes a “spectroscopic detector” for discriminating between different narrow-line luminescent responses.
U.S. Pat. No. 3,412,245 adds the decay-time characteristics of the luminescence to the encoding factors. In this way, rare-earth based luminescents, having decay times of the order of milliseconds, can be distinguished from much more rapidly decaying organic fluorescent material. The discrimination is done via excitation with sinusoidally modulated or pulsed UV light sources, using variable modulation or pulse frequency, in conjunction with spectral separation of the different emission wavelengths.
U.S. Pat. No. 3,582,623 and U.S. Pat. No. 3,663,813 reveal further developments of spectroscopic detecting equipment for luminescent features.
U.S. Pat. No. 3,650,400 describes the use of a pulsating light source, in conjunction with synchronous detection at the pulsating frequency (“lock-in” principle), to suppress the influence of ambient light. By this means, the detector is only sensitive to the luminescent's proper response. The principal shortcoming of methods of the prior art, which rely on a determination of the material's modulation-transfer function, is their inherent slowness. For these reason they are not normally implemented on high-speed authenticating machines.
U.S. Pat. No. 4,047,033 describes the use of an up-converting luminescent material for security purposes, as well as corresponding detecting equipment. Detection relies on excitation with a GaAs IR-LED, emitting at 950 nm wavelength in continuous or pulsed mode, combined with spectroscopic identification of the luminescent emission. Reference is made to indirect means, by measuring pulse dephasing, for assessing the characteristic rise and decay times of the luminescent's response. This method is, however, strongly affected by variations in luminescence intensity, and therefore not easy to implement in practice.
Another method of the prior art, suited for high-speed authentication, relies on pulse-excitation of a moving test sample on a conveyor belt. After passing the UV excitation source, the intensity of the induced luminescence decays according to the intrinsic decay characteristics of the material. One or several photodetectors, placed at determined distances from the UV source along the conveyor belt, are used to assess specific points of said decay characteristics. The main drawback of this method is its limitation to such phosphorescent materials which have characteristic luminescence decay times of the order of 50 milliseconds. This limitation is a consequence of the mechanical constraints (conveyor belt speed) of the detecting process.
It is an object of the present invention to provide a method, a device and a security system which overcome the shortcomings of the prior art. In particular, the invention shall permit a rapid sampling of a luminescence decay characteristic and shall be, therefore, suitable for high-speed machine reading applications.
Furthermore, the invention shall allow a wide choice of up-or down-converting phosphorescent materials, having decay times from the sub-microseconds to the ten milliseconds range or longer. A further particular object of the invention is to render the authentication process more reliable by compensating for alterations of luminescence intensity, which may occur due to changes in the luminecent marking itself (ageing, dirtiness) or in the measuring equipment.